Post landslides, Nepal high dam debate continues

The landslide in Nepal, which has pent up the Bhote Kosi, some 100 kms north-east of Kathmandu, has again revived the debate, whether a high dam in Barahkshetra as suggested by a section of experts in the last three decades, would help solve the problem of apocalyptic floods in the north Bihar flood plains.

The first such plan was mooted in 1945 by the then viceroy of India, Lord Wavell. However, AN Khosla, chairman of the central water and power commission, submitted the first scheme for river Kosi, which envisaged construction of a high dam for flood control and a barrage for irrigation in early 1950s. This was expected to moderate the flood flow to 5,600 cumecs with potential to generate 250MW of power at a cost of Rs 177 crore.

The scheme was put up before an advisory committee, which dropped it because the proposed dam was in a highly seismic zone, the 20 year construction period was too long and the huge amount of cement required was beyond the capacity of Indian companies. The advisory committee, instead recommended a low detention reservoir at Belka and a marginal embankment on the eastern side.

However, even this idea was dropped as it was felt that it would be impossible to build a dam of about 130 metres on a basically sand foundation. Eventually, the present Kosi project without inclusion of the high dam was visualised in 1954.

It was in 1974, that following the failure of the embankments to protect Bihar and flooding encroachment extending from 2.47 mha to 4.05 mha in the 20-year since 1954, that the Kosi board was constituted with engineer in chief Kanwar Sain as its head. The board after examining the situation said a high dam in Barahkshetra in Nepal to dam and control the various tributaries of the river could help solve the flooding.

It believed, it would also have helped to solve the problem of silting witnessed upstream and downstream of the Bhimnagar barrage, near Birpur and reduce the highest peak floods to safe limits while providing power at a reasonable cost and allow use of stored water for irrigation in Nepal and India.

However, the observation of the Kanwar Sain board was countered by the chief engineers of irrigation and power departments. They had warned, the idea to construct a high dam to provide ample storage of silt and moderating maximum flood levels from 23,000 cumecs to 5,600 cumecs was fraught with danger since the various sites suggested for the dam at Barahkshetra, Belka and Kothar in Nepal were very close to the epicentre of past disastrous earthquakes and high seismic activity.

They also said, the relief provided by the Belka and Kothar dam sites via silt storage would not be commensurate to the expenditure. A geological survey of India survey, also concluded, that a 155 metre dam, as proposed across the Kosi at Kothar will silt up within a short period because of the heavy landslides in the catchment area of the Sun Kosi and Tamur rivers, as has happened now.

Apart from the seismic factor and certainty of high siltation rates, the effectiveness of a flood moderation dam may also be low because of the large free catchment area downstream.

It was pointed out, that if indeed there was moderation of the frequency and intensity of floods, a further encroachment of flood plains would take place in the lower basins in Darbhanga, Khagaria, Saharsa and Purnia districts.

Consequently, a situation may arise, when even a moderate increase in water levels during peak flow in conjunction with water flows from uncontrolled areas could lead to damages exceeding the pre-reservoir period.

The irrigation potential of proposed dams also raise several questions, especially in the context of the under-utilisation of existing groundwater potential in Kosi plains. As per one estimate, 35% of the total cultivable land gets waterlogged in north Bihar for a period ranging from three months to a year in different districts. Hence the Bihar governmentfs insistence for storage reservoirs in Nepal also does not hold water, experts say.